This invention relates generally to optical phase conjugation and, more particularly, to optical phase conjugation using stimulated Brillouin scattering (SBS). It has been recognized for some time that phase conjugation of light waves has some important applications, such as the removal of phase aberrations caused by the passage of a light beam through a distorting or phase-aberrating medium.
There is extensive literature on the subject of phase conjugate optics and the use of phase conjunction for the compensation of phase aberrations. A summary of the history and principles of phase conjugate optics is provided in a paper entitled "Phase Conjugate Optics and Real-Time Holography," by Amnon Yariv, IEEE Journal of Quantum Electronics, Vol. QE-14, No. 9, Sept., 1978, pp. 650-60.
Simply stated, a phase conjugation cell functions as a reflector with a special and useful property. When an incident light wave is focused into the cell, the reflected wave that emerges is the complex conjugate of the incident wave. The practical consequence of the phase conjugation is that the retro-reflected wave is as if it were "time-reversed" with respect to the incident wave. For example, if an incident wave, after passing through a distorting medium, has a bulge in its wavefront, representing a phase-lagging condition at a particular region of the front, this will be reflected as an opposite bulge, i.e. a phase-leading condition, in the same region of the reflected wavefront. If the reflected wavefront then traverses the same distorting medium that caused the original bulge in the incident wavefront, the reflected wave will emerge from the distorting medium as an undistorted wave.
Phase conjugation can be achieved through the use of stimulated Brillouin scattering. When a light beam of sufficient energy is focused into a stimulated Brillouin scattering (SBS) cell, there is reflection of a phase-conjugated beam. Unfortunately, however, the SBS phase conjugation process has a relatively high threshold of operation. That is to say, relatively high powers are required before the phase conjugation process even begins to work. In many high-power laser applications, consideration is being given to operation at power levels many times the threshold, for most efficient operation. If the threshold is too high, operation at multiples of the threshold becomes impractical. Consequently, SBS phase conjugation has been limited for use at relatively low average power levels, using short pulses of operation.
In the past, optical waveguides have been used to increase the effective interaction length in SBS phase conjugation. However, this approach is limited to relatively low powers and cannot be scaled up for use with high-power lasers.
The availability of a lower threshold SBS cell would facilitate the use of SBS phase conjugation for higher average power situations, and would allow the use of laser sources of lower power. In addition, a lower threshold SBS cell would reduce problems due to competing processes, such as thermal blooming, and would increase the overall efficiency of a system including SBS cells and laser sources.
It will be appreciated from the foregoing that there is a clear need for a new technique for reducing the threshold of operation of an SBS phase conjugation cell. The present invention is directed to this end.